When the Challenger space shuttle exploded in January, 1986 subsequent investigations, as reported in The New York Times of Dec. 8, 1986, page B15, indicated that the root of the problem was the booster rockets four large steel-walled segments. These 27 foot long segments had thick rubber insulation between casing and fuel to keep the searing heat of burning propellant from melting the walls. The temperature of this heat can reach upwards of 5800.degree. F. Specially prepared rubbers are vulcanized to the inside of the metal casing utilizing pressure and heat to ensure the necessary bonding. However, no insulation covered or could cover the precise areas where the segments fit together. Instead, they were locked together by a tongue-and-groove joint and additionally protected from the leakage of hot gases by a pair of rubbery O-shaped seals or rings.
Unfortunately, pressures in the rocket forced the joint open which allowed hot gases and flames to contact the O rings. As further reported, an additional problem resulted from the exceptionally cold weather on the morning of the launch, which kept the O rings from expanding properly and thereby allowed flames to leak past them outside the rocket and disastrously caused the explosion in the huge adjacent fuel tank full of liquid hydrogen.
It has been recognized that segmenting of the rocket casing makes it difficult to protect the joints between the sections from the searing rocket flames. Nevertheless, segmented rocket casings have been used worldwise because they can be manufactured far from launch and shipped in pieces to launching sites where they are assembled. Monolithic rocket casings had been proposed heretofore, but it was believed that by casting giant rockets in sections the segments are much easier to ship and easier to inspect than monolithic casings.
A monolithic steel rocket casing involving welding seams and joints was designed by the Aerojet Solid Propulsion Company in accordance with a report in The New York Times of Dec. 7, 1986. The report indicates that Aerojet, one of four bidders for a U.S. government contract, faulted the rubbery seals of segmented rockets while the other three bidders presented segmented rocket casings.
Although at one time there was some concern that the fuel in monolithic rocket casings would be difficult to inspect for bubbles and cracks and might therefore contain hidden paths allowing a dangerous quick burn toward the metal casing, improved techniques and expertise have essentially eliminated these doubts.
Another concern about the monolithic casings heretofore available arose because the welds cannot be trusted under the elevated temperatures and pressures encountered during use, as evidenced by several failures when the weld ruptured. Moreover, at present there is no reliable inspection method for fail safe welds.
It would be desirable therefore to eliminate the joints and the concomitant need for O rings or similar gaskets as well as the need for welding in the manufacture of metallic rocket casings. It would also be desirable to have available a method for readily manufacturing seamless, monolithic rocket casings in the general area of launching or sufficiently close to avoid any possible transportation problems. Moreover, it would be desirable to have available a method which can readily be employed for producing a variety of single piece (i.e., no seams or joints) casings essentially on site, obviating the recognized multiple handling and transportation problems.
Referring again to rocket casings specifically, it appears that there are two possible methods of manufacture. The one discussed in detail above involves casting and forging steel with subsequent heat treatment and quenching steps in order to obtain the desired physical characteristics. An attempt to cast rocket casings in one piece, e.g., 108 feet by 12 feet in diameter, is a monumental undertaking and that is why small segments are prepared and either joined together as described above, or welded together.
The other method available at the present time is to filament-wind graphite or glass fibers coated with epoxy resin around pre-formed metallic cylinders. However, this method has been found to be too limited in use for fabrication of large size casings which would be difficult to maneuver in all directions. In addition, one of the objects of the present invention is to prepare relatively thin-walled, large casings with high tensile strengths rather than thick-walled casings which would obviously have size limitations.
It will be recognized that the manufacture of fail safe booster rocket casings is of the utmost importance for space exploration and space defense programs. As evidenced by recent failures, there is a need for superior casings, and especially for casings that do not have the potential weak link joint seals required when assembling casing segments that have been separately manufactured, filled with rocket fuel and then transported to the launch site. As will be further understood, merely to substitute monolithic steel casings for the present steel segments poses formidable problems because of the enormous weights involved. Thus, manufacturing operations for previously proposed monolithic casings are certain to be more complicated and more expensive than those for casing segments. In addition, transportation and hoisting problems would be enormous. Consequently any redesign of booster rocket casings must not only avoid the present problems with segmented casings but also the obvious manufacturing problems posed by producing seamless casings that must combine relatively great height with great strength.